The steam generators of pressurized-water nuclear reactors comprise a bundle of tubes whose ends are crimped into a tube plate whose thickness is generally greater than 500 mm. The tubes of the bundle, which are made of nickel alloy, have an internal diameter of slightly less than 20 mm.
After the steam generator has been operating for a certain time, certain tubes of the bundle may exhibit more or less pronounced damage, and in particular fissures across their wall. In this case, the tube no longer provides a perfect separation between the pressurized primary water in contact with its inner wall and the feed water of the steam generator in contact with its outer surface. The tube must therefore either be plugged to take it out of service, or a repair of the tube wall must be performed in its fissured region.
The tubes of a steam generator may be repaired by lining, this operation consisting in introducing a liner into the tube and in ensuring its fastening to the inner wall of the tube, on each side of the fissured region.
The fastening of the liner, generally by diametral expansion and by expanding by rolling, must be supplemented by welding the ends of the liner to the tube, over their entire periphery, so as to provide a perfectly sealed connection between the tube and the liner. This operation may be performed by using a rotary TIG welding device comprising a welding head which is introduced into the tube in the region of the ends of the liner which has been fastened beforehand into the tube. The welding head comprises a carrier to which are fastened a component permitting the head to be guided in the tube and an electrode arranged radially in relation to the head and to the tube into which the head is introduced.
The welding head is mounted so that it can rotate around its axis at the end of a welding probe which may be constructed in a completely rigid form or, on the contrary, may consist of flexible sections and rigid sections. The welding probe comprises conduits for feeding the welding head with protective inert gas and with cooling water, a cable for supplying a welding current to the electrode, and means for rotating the welding head and its electrode about the tube axis.
In the case of rotary welding inside the steam generator tubes of a pressurized-water nuclear reactor, the welding probe must be introduced into each of the tubes which is being lined, through the entry face of the tube plate, inside the waterbox of the steam generator.
This waterbox, hemispherical in shape, comprises two compartments each of which is accessible through a manhole.
The space inside the waterbox, which comes into contact with the primary water, shows some radioactivity, with the result that operations inside the waterbox must be of very low duration or must be controlled from outside the waterbox.
Furthermore, the introduction of a rigid tool of some length into the tubes of the bundle is possible only in the middle part of this bundle, since, as a result of the hemispherical shape of the waterbox, the space available under the tube plate quickly becomes too small in the case of the tubes situated near the periphery of the bundle, to make it possible to ensure that the tool introduced is positioned vertically.
Proposals have therefore been made for operating procedures which are adapted to the case of the peripheral tubes of the bundle, and corresponding devices which make it possible to perform the various operations involved during the lining operation.
However, until now no rotary TIG welding probe adapted to the case of the peripheral tubes of a steam generator has been proposed.
The majority of the devices proposed in the case of operations in the tubes of a steam generator, and particularly in the peripheral tubes, comprise rigid parts and flexible parts connected together, forming a movable tool which can be moved in a guide tube providing the connection between the outside of the waterbox and the inlet of the steam generator tube in which the operation takes place. By virtue of such movable and flexible tools, operations or checks can be carried out in steam generator tubes at any height inside these tubes.
Until now, it has not been possible to design a welding probe offering the same advantages as the devices mentioned above and permitting an operation in order to weld a liner at any height inside a steam generator tube.
In fact, the welding probe must ensure a highly accurate positioning of the welding head in the tube and a perfect rotational guidance of this head during the welding. These conditions are difficult to achieve with an assembly comprising flexible parts and rigid parts and driven in rotation by a driving means situated outside the waterbox.
There is a known, remote-controlled welding device which may be employed for fastening liners in steam generator tubes, and which comprises a rigid welding head fastened to the end of a flexible tubing which carries internally side the means for feeding the welding head, and by means of which the positioning of the welding head can be carried out in the region of any tube of the bundle. This welding unit employs a carrier device of known type, which is fastened under the tube plate and which makes it possible to support and to position in the region of any tube of the bundle the end of a flexible guide tube in which the welding head moves together with its flexible connection tube.
Also fastened to the carrier device is a means permitting the welding head to be moved inside a steam generator tube and to be positioned in a perfectly determined position inside the tube, together with a set of bearings and driving means permitting the welding head to be rotated inside the tube.
The use of a device of this kind is limited, since the rigid welding head must continually remain in engagement with the bearings and with driving means carried by the carrier device situated under the tube plate.
The length of the rigid welding head must be limited to a relatively low value, above all when the ability to perform the operations in the tubes situated at the periphery of the bundle is desired.
It is therefore not possible to perform welding operations at a considerable height inside the tube.
The welding assembly according to the prior art can, in fact, be employed only for performing welding operations inside the tube plate or, at best, in the vicinity of its outlet face.